Relatively pure forsterite (Mg2SiO4) has been determined to have a melting point of as much as 1890° C. ±20° C. As such, it qualifies as being among the types of basic super refractories needed for many applications. For example, it finds application as a material of construction for molten metal flow control devices used in the steel industry, and for linings and coatings employed in steel ladles which are in turn used to collect molten metal during alloy fabrication and smelting. The steel industry historically used basic materials such as magnesite and dolomite because of the limiting factors of corrosion resistance and temperature resistance of alumino-silicate refractories, which are less than that of the minerals based upon MgO compounds. Therefore, forsterite could provide refractory linings with properties superior to other commonly used materials, but with unique physical and chemical properties not available from the other minerals.
Forsterite (Mg2SiO4) does not occur naturally in a pure form, but, in isomorphous combination with fayalite (Fe2SiO4). Tremendous quantities appear as dunite olivine ((Mg,Fe)2SiO4), in the state of Washington and in the Appalachian Mountain areas of North Carolina and Georgia. Recently, large quantities of olivine have arisen, in new geographic regions, but methods are needed to increase the refractoriness of the final forsterite. To produce the forsterite from olivine ((Mg,Fe)2SiO4), in the past the olivine has been melted in an electric arc-resistance furnace, with simultaneous reduction of the iron (fayalite) components, to form a product consisting essentially of crystalline forsterite.
The mineral commonly called olivine consists of an isomorphous combination of forsterite ((Mg)2SiO4) and fayalite (Fe2SiO4), which within the range of 5-25% of fayalite is often termed chrysolite. In addition to secondary minerals formed by alteration, such as serpentine and chlorite, olivine is commonly associated with several primary minerals, including chromite, bronzite and enstatite. Mineralogically the group term “olivine” includes orthosilicates of divalent bases crystallizing in the orthorhombic system. This larger group may be represented by the formula (Mg, Fe, Mn, Ca)(Mg, Fe, Mn, Zn, Pb)SiO4, with the following recognized end compounds: Forsterite, MG2SiO4; fayalite, Fe2SiO4; tephroite, Mn2SiO4; monticellite, CaMgSiO4; glaucochroite, CaMnSiO4; and larsenite, PbZnSiO4.
During the fusion of olivine to produce the forsterite, a reducing agent, most commonly, carbon is present. The main impurities within the olivine that need to be removed during the course of the high temperature fusion are the various iron oxides FeO, Fe2O3 and Fe3O2. As the fusion progresses, the molten ore is converted to the form of a molten glass. In this amorphous state, the iron oxides along with silicon oxide are easily reduced to a ferrosilicon alloy, i.e., an agglomeration of metallic iron and metallic silicon. The ferrosilicon alloys are separated from the oxide melt and collected as a by-product for use in other applications. The remaining molten oxide which comprises predominantly magnesium silicate is poured out of the furnace into molds. The melt within those molds is then allowed to cool, forming crystalline forsterite.
While the carbon reducing agent readily reduces the iron components of the melt, there exists unwanted excess silica which tends to weaken the load bearing strength of the refractory but is not expelled during the reduction. Additional carbon up to 5% lowered the undesired silicon oxide SiO2 to less than 2%. Also, it was previously found that additions of magnesium oxide and calcium oxide further assisted in removing the silica (SiO2). However, this inhibited the reduction of the iron. Accordingly, the carbon reducing content has continued to remain to be believed critical to the process.
Previously, the crystalline forsterite has been rather weak and friable with average crystal sizes being smaller than 200 microns. A method for promoting the growth of larger crystals during the course of forsterite production would be a welcomed advancement in the art.